Add Javadoc indicating which server version supports explain

JAVA-3909

Author: jyemin

driver-legacy/src/main/com/mongodb/DBCollection.java

@@ -1252,6 +1252,7 @@ public Cursor aggregate(final List<? extends DBObject> pipeline, final Aggregati
      * @return the command result.  The explain output may change from release to release, so best to simply log this.
      * @mongodb.driver.manual core/aggregation-pipeline/ Aggregation
      * @mongodb.driver.manual reference/operator/meta/explain/ Explain query
+     * @mongodb.server.release 3.6
      */
     public CommandResult explainAggregate(final List<? extends DBObject> pipeline, final AggregationOptions options) {
         AggregateOperation<BsonDocument> operation = new AggregateOperation<BsonDocument>(getNamespace(), preparePipeline(pipeline),

driver-legacy/src/main/com/mongodb/DBCursor.java

@@ -335,7 +335,7 @@ public DBCursor maxTime(final long maxTime, final TimeUnit timeUnit) {
      * @return a {@code DBObject} containing the explain output for this DBCursor's query
      * @throws MongoException if the operation failed
      * @mongodb.driver.manual reference/command/explain Explain Output
-     * @deprecated Replace with direct use of the explain command using the runCommand helper method
+     * @mongodb.server.release 3.0
      */
     @Deprecated
     public DBObject explain() {